Method and apparatus for automated tracking of non-linear vessel movement using MR imaging

ABSTRACT

A system and method is disclosed for tracking a moving object using magnetic resonance imaging. The technique includes acquiring a scout image scan having a number of image frames and extracting non-linear motion parameters from the number of image frames of the scout image scan. The technique includes prospectively shifting slice location using the non-linear motion parameters between slice locations while acquiring a series of MR images. The system and method are particularly useful in tracking coronary artery movement during the cardiac cycle to acquire the non-linear components of coronary artery movement during a diastolic portion of the R-R interval.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to an improved method foracquiring magnetic resonance images (MRI) of moving objects, and moreparticularly to, a method and apparatus to improve the efficiency ofmagnetic resonance coronary angiography (MRCA).

[0002] When a substance such as human tissue is subjected to a uniformmagnetic field (polarizing field B₀), the individual magnetic moments ofthe spins in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, or “longitudinalmagnetization”, M_(Z), may be rotated, or “tipped”, into the x-y planeto produce a net transverse magnetic moment M_(t). A signal is emittedby the excited spins after the excitation signal B1 is terminated andthis signal may be received and processed to form an image.

[0003] When utilizing these signals to produce images, magnetic fieldgradients (G_(x) G_(y) and G_(z)) are employed. Typically, the region tobe imaged is scanned by a sequence of measurement cycles in which thesegradients vary according to the particular localization method beingused. The resulting set of received NMR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

[0004] Moving objects are particularly difficult to image, especially ifan imaging plane is set in space with the object moving in and out ofthe imaging plane. Such imaging is especially difficult when a secondperiodic motion is added thereto. For example, imaging of objects in asubject which is breathing causes a periodic motion of internalstructures, which is also further complicated by the beating motion ofthe heart if the structure is near the heart.

[0005] Acquisition of images during an end-expiratory breath-hold iscommonly employed to minimize respiratory artifacts, whileelectrocardiography (ECG) gating can effectively freeze cardiac motion.Breath-held, ECG-gated two-dimensional (2D) CMRA can be accomplishedusing several imaging strategies, the most common being a 2D fastgradient-echo sequence segmented k-space acquisition (fgre). Twostrategies for 2D CMRA are acquisition of the same slice over the entirecardiac cycle (traditional “CINE”) or acquisition of multiple sliceswith differing cardiac phases, typically acquired during mid-diastole.The prior art has successfully developed coronary artery imaging duringthe systolic phase, where a single image is acquired per acquisition.While such methods require that segments of the coronary artery beconstrained within the plane of the prescribed slices, they do not makeany implicit assumptions regarding the motion of the coronary arteriesover the entire R-R interval. The visualization of thevessel-of-interest is therefore only ensured in a few frames.

[0006] Since there is substantial motion of the right coronary artery(RCA) and the left anterior descending (LAD) artery (in the order of 2cm or more) during the cardiac cycle, the imaging efficiency (i.e.,percentage of images containing a significant length of thevessel-of-interest) of these sequences is low. This implies thatvisualization of the vessel in its entirety generally requires severalrepeated breath-holds covering overlapping or contiguous slicelocations, prolonging the scan times, which is generally not acceptablefor patients with coronary artery disease.

[0007] The prior art proposed a method of tracking the motion of thecoronary arteries prospectively across the cardiac cycle as a functionof the delay from the cardiac trigger to improve the imaging efficiency.By adjusting the slice position as a function of cardiac phase, multipleimages can be acquired in a single breath-hold, effectively tracking thevessel as a function of cardiac phase. This method reported an improvedefficiency for the vessel tracking sequence compared to the multi-slicesequence. However, the prior art assumed a linear model for the motionof the vessel from its end-systolic to its end-diastolic position andback. While this linear model is often accurate in systole, duringdiastole, especially for the RCA, it is not. It has been found that themotion in diastole does not fit the linear model. As a consequence, thevisualization efficiency in the diastolic phase, where the vessel movesthe least, was less than optimal.

[0008] It would therefore be desirable to have a method and apparatus toimprove the efficiency of acquiring MR images of a moving object byaccurately, and automatically, tracking the moving object over amovement cycle. In particular, it is desirable to improve the efficiencyof ECG-gated MRCA by accurate and automatic tracking of coronary vesselmotion over the cardiac cycle.

SUMMARY OF THE INVENTION

[0009] The present invention relates to a system and method for imaginga moving object using magnetic resonance imaging (MRI) that utilizesnon-linear object tracking to improve the efficiency of the imaging thatsolves the aforementioned problems.

[0010] The invention includes a technique that is a hybrid betweenmulti-phase, single slice and single phase, multi-slice techniques. Asapplied to MR coronary angiography, the invention includes trackingcoronary artery movement during a cardiac cycle as the coronary arterymoves between two excursion positions in the R-R interval. The techniqueincludes determining a function of the coronary artery movement havingat least a non-linear component that represents movement during adiastolic portion of the R-R interval. Slice position acquisition isadjusted using the determined function and MR data is acquired duringeach such adjustment. The resulting MR images reconstructed haveincreased imaging efficiency using the MR data acquired.

[0011] In accordance with another aspect of the invention, a method ofimaging a moving object using MRI includes acquiring a scout imagingscan having a number image frames, and extracting non-linear motionparameters from the number of image frames of the scout image scan. Themethod next includes acquiring a series of MR images while prospectivelyshifting a slice location using the non-linear motion parameters betweenslice locations. The method may take advantage of linear parameters aswell as non-linear parameters to track the moving object.

[0012] Yet another aspect of the invention includes an MRI apparatus totrack and image a moving object-of-interest that includes a magneticresonance imaging system having an RF transceiver system and a pluralityof gradient coils positioned about the bore of a magnet to impress apolarizing magnetic field. An RF switch is controlled by a pulse moduleto transmit RF signals to an RF coil assembly to acquire MR images. Acomputer is programmed to acquire a series of scout scans of theobject-of-interest using a multi-phase, single slice acquisition pulsesequence as the object-of-interest moves from one position to another. Areference position in a scout scan is identified and the movement of theobject-of-interest is tracked by isolating the reference positionin eachscout scan in the series of scout scans. A set of motion parameters aredetermined and stored that include non-linear parameters to accuratelyprospectively track movement of the object-of-interest.

[0013] In accordance with yet another aspect of the invention, acomputer program is disclosed having a set of instructions which, whenexecuted by a computer, cause the computer to track coronary arterymovement during a cardiac cycle as the coronary artery moves between twopositions. The computer is also caused to determine movement parametersof the coronary artery movement that has at least a non-linear componentthat represents movement during a diastolic portion of a R-R interval,and then stores the movement parameters in memory. Slice positionacquisition is then adjusted using the stored movement parameters andimage data is acquired during each such adjustment such that an image isreconstructed having increased imaging efficiency.

[0014] The imaging efficiency is defined as the percentage of the sliceswhere more than 30 mm of the vessel is visualized. Since vessel trackingis preferably implemented on a spiral gradient-echo pulse sequenceachieving sub-millimeter spatial resolution, as well as a highersignal-to-noise ratio (SNR), a significant improvement in the efficiencyof the vessel tracking sequence was achieved. When using across-correlation algorithm for vessel tracking, imaging efficiency isincreased even further. Additionally, a higher flip angle can be used toobtain improved image quality since the sequence repetition times arehigher in the spiral sequence. The software implementing this techniquecan be integrated into the operator's console to achieve real-timeprescription.

[0015] Various other features, objects and advantages of the presentinvention will be made apparent from the following detailed descriptionand the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings illustrate one preferred embodiment presentlycontemplated for carrying out the invention.

[0017] In the drawings:

[0018]FIG. 1 is a schematic block diagram of an NMR imaging system foruse with the present invention.

[0019]FIG. 2 is a diagram illustrating the maximum excursion positionsof a coronary artery.

[0020]FIG. 3 is a prior art graph of artery displacement versus time fora linear model of artery motion over the cardiac cycle.

[0021]FIG. 4 is a graph showing displacement of the RCA as a function ofposition in the R-R interval for various subjects.

[0022]FIG. 5 is a flow chart of a technique employed in the presentinvention.

[0023]FIG. 6 is a graph representing relative displacement versus sliceacquisition in an R-R interval.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] Referring to FIG. 1, the major components of a preferred MRIsystem 10 incorporating the present invention are shown. The operationof the system is controlled from an operator console 12 which includes akeyboard or other input device 13, a control panel 14, and a display 16.The console 12 communicates through a link 18 with a separate computersystem 20 that enables an operator to control the production and displayof images on the screen 16. The computer system 20 includes a number ofmodules which communicate with each other through a backplane 20 a.These include an image processor module 22, a CPU module 24 and a memorymodule 26, known in the art as a frame buffer for storing image dataarrays. The computer system 20 is linked to a disk storage 28 and a tapedrive 30 for storage of image data and programs, and it communicateswith a separate system control 32 through a high speed serial link 34.The input device 13 can include a mouse, joystick, keyboard, track ball,touch screen, light wand, voice control, or similar device, and may beused for interactive geometry prescription.

[0025] The system control 32 includes a set of modules connectedtogether by a backplane 32 a. These include a CPU module 36 and a pulsegenerator module 38 which connects to the operator console 12 through aserial link 40. It is through link 40 that the system control 32receives commands from the operator which indicate the scan sequencethat is to be performed. The pulse generator module 38 operates thesystem components to carry out the desired scan sequence and producesdata which indicates the timing, strength and shape of the RF pulsesproduced, and the timing and length of the data acquisition window. Thepulse generator module 38 connects to a set of gradient amplifiers 42,to indicate the timing and shape of the gradient pulses that areproduced during the scan. The pulse generator module 38 also receivespatient data from a physiological acquisition controller 44 thatreceives signals from a number of different sensors connected to thepatient, such as ECG signals from electrodes attached to the patient.And finally, the pulse generator module 38 connects to a scan roominterface circuit 46 which receives signals from various sensorsassociated with the condition of the patient and the magnet system. Itis also through the scan room interface circuit 46 that a patientpositioning system 48 receives commands to move the patient to thedesired position for the scan.

[0026] The gradient waveforms produced by the pulse generator module 38are applied to the gradient amplifier system 42 having G_(x), G_(y), andG_(z) amplifiers. Each gradient amplifier excites a correspondingphysical gradient coil in an assembly generally designated 50 to producethe magnetic field gradients used for spatially encoding acquiredsignals. The gradient coil assembly 50 forms part of a magnet assembly52 which includes a polarizing magnet 54 and a whole-body RF coil 56. Atransceiver module 58 in the system control 32 produces pulses which areamplified by an RF amplifier 60 and coupled to the RF coil 56 by atransmit/receive switch 62. The resulting signals emitted by the excitednuclei in the patient may be sensed by the same RF coil 56 and coupledthrough the transmit/receive switch 62 to a preamplifier 64. Theamplified MR signals are demodulated, filtered, and digitized in thereceiver section of the transceiver 58. The transmit/receive switch 62is controlled by a signal from the pulse generator module 38 toelectrically connect the RF amplifier 60 to the coil 56 during thetransmit mode and to connect the preamplifier 64 during the receivemode. The transmit/receive switch 62 also enables a separate RF coil(for example, a surface coil) to be used in either the transmit orreceive mode.

[0027] The MR signals picked up by the RF coil 56 are digitized by thetransceiver module 58 and transferred to a memory module 66 in thesystem control 32. When a scan is completed, an array of raw k-spacedata has been acquired in the memory module 66. As will be described inmore detail below, this raw k-space data is rearranged into separatek-space data arrays for each image to be reconstructed, and each ofthese is input to an array processor 68 which operates to Fouriertransform the data into an array of image data. This image data isconveyed through the serial link 34 to the computer system 20 where itis stored in the disk memory 28. In response to commands received fromthe operator console 12, this image data may be archived on the tapedrive 30, or it may be further processed by the image processor 22 andconveyed to the operator console 12 and presented on the display 16.

[0028] The present invention includes a method and system suitable foruse with the above-referenced MR system, or any similar or equivalentsystem for obtaining MR images, that when used with a fast-imagingtechnique, to acquire a set of scout frames, is used for automaticallytracking and mapping object motion generally, and more specifically, toautomatically map vessel trajectory to improve artery tracking in MRAngiography (MRA).

[0029] In accordance with the invention, in order to acquire MR imagesof a moving coronary artery in a specific scan plane, or plane ofacquisition, it must first be prescribed (either graphically or byproviding the computer with the location of at least two points of theacquisition or scan plane). The prescription of a scan or acquisitionplane is customarily performed using a scout scan that is in a planeorthogonal to that of the acquisition or scan plane. From a multi-phaseCINE acquisition at the same location and rotational orientation as theplane of the scout scan, the motion of the artery or any other vessel inthe desired plane of acquisition or scan plane, during the cardiaccycle, can be observed. The maximum excursion or displacement of thevessel during the cardiac cycle, or R-R interval, can then beidentified.

[0030] Referring to FIG. 2, there is shown a representation of suchscout image, which depicts a coronary artery 110, such as a rightcoronary artery, within a heart 112. The artery 110 is located at oneextreme position at the beginning of the cardiac cycle, i.e., thediastolic position of maximum excursion or end-diastolic position 114,progresses to the other extreme position, i.e. the systolic position ofmaximum excursion or end-systolic position 116, and returns to thestarting location, i.e., end-diastolic position 114, at the conclusionof the cardiac cycle. The position of the coronary artery 110 is thusconstrained to a region 118, lying between the two positions 114 and116. Accordingly, it is unnecessary to prescribe imaging slice locationsbeyond the bounds determined by such maximum excursion positions 114 and116, as observed from the CINE scout scan. Moreover, the maximumexcursion of the artery 110, i.e., the displacement D_(max) betweenmaximum excursion positions 114 and 116, can be readily determined bysimply measuring such displacement on the CINE scout image, by means ofcalipers or the like. The multi-phase scout acquisition will alsoprovide information as to the obliquity of the coronary artery. Thescout scan can be carried out, for example, in accordance with an MRsequence which is conventionally available on MR imaging products of theGeneral Electric Company, and which is referred to thereby as a CINEscout view sequence. As is known, a CINE sequence comprises a timeresolved pulse sequence, i.e., a succession of views acquired at thesame location but at different times. It will be readily apparent that anumber of other MR techniques are available for use in determining thedisplacement between maximum excursion positions 114 and 116.

[0031] Once the region 118, which defines the possible locations of thecoronary artery 110, and the maximum displacement D_(max) have beendetermined, an algorithm can be derived to estimate the position of thecoronary artery 110, or other vessel as a function of time or time delayfollowing commencement of the cardiac cycle, or detection of the cardiacelectrical R-wave trigger. MR data may be acquired throughout thecardiac cycle, with the slice excitation position adjusted as a functionof the time delay of the RF pulse of the MR imaging sequence, likewisefrom the cardiac R-wave trigger. Thus, the MR scan or image acquisitionplane, which corresponds to the position of a slice, can be adjusted orsteered to track the location of the coronary artery 110, as the arterymoves through the cardiac cycle.

[0032] In view of the effort to track the MR data acquisition with themotion of the artery 110, it is desirable to fit as many images aspossible into the R-R interval. By using a segmented k-space approach,the maximum number of slice locations that will be acquired can beexpressed as: $\begin{matrix}{{nslices}_{\max} = \frac{RR\_ time}{{vpsxTR} + {cs\_ sattime}}} & \text{Eqn.~~1}\end{matrix}$

[0033] where nslices_(max) is the maximum number of images per R-Rinterval, vps is the number of views or k-space lines acquired persegment (or per R-R interval), cs_sattime is the time needed to play outa fat suppression pulse, and TR is the sequence repetition time.

[0034] Referring further to FIG. 2, there is shown the data acquisitionscan plane at a location 120, for a slice taken through end-diastolicposition 114 at the beginning of the cardiac cycle. There is furthershown the scan plane at a position 122, for a slice taken through theend-systolic position 116. The scan plane is at respective locations 124and 126 for slices acquired at different locations within region 118. Itwill be readily apparent that a slice at a given location should beexcited at a time during the cardiac cycle such that the coronary artery110, or at least a substantial portion thereof, will also be at thegiven location at the time of excitation.

[0035] As is well known to those of skill in the art, the motion of acoronary artery is generally different during systole and diastole. Thetime for systole (t_(systole)) is calculated in milliseconds as follows:

t _(systole)=546−(2.1×HR)  Eqn. 2

[0036] where HR is the heart rate in beats per min. From thisexpression, the time of diastole (t_(diastole)) in milliseconds can becalculated as follows: $\begin{matrix}{{t_{diastole} = {\frac{60{,000}}{HR} - t_{systole}}},} & \text{Eqn.~~3}\end{matrix}$

[0037] The numerical parameters respectively used in Eqns. (2) and (3)are derived from teachings well known to those of skill in the art. Anexample of such a teaching is Bacharach S L, Bonow R O, Green M V,Comparison of fixed and variable temporal resolution methods forcreating gated cardiac blood-pool image sequences, J. Nucl. Med. 1990;vol. 31: 38-42.

[0038] Referring to FIG. 3, there is shown a prior art time-displacementplot or curve, which represented displacement of artery 110 during theR-R interval, as the artery moves between end-diastolic position 114 andend-systolic position 116. Such prior art assumed a linear model forsuch motion, so that velocities v_(systole) and v_(diastole), thevelocities during systolic and diastolic motion, respectively, hadlikewise been assumed to be constant. In FIG. 3, D_(dias) indicates theposition of coronary artery 110 identified in the scout scan asend-diastolic position 114, and D_(sys) indicates the end-systolicposition 116 identified thereby. Artery 110 is at D_(dias) at thebeginning of the cardiac cycle, and then moves in systole with avelocity v_(systole) given by the following expression: $\begin{matrix}{V_{systole} = \frac{\left( {{D_{sys} - D_{dias}}} \right)}{I_{systole}}} & \text{Eqn.~~4}\end{matrix}$

[0039] After the artery reaches end-systole position D_(sys), it movesin diastole, in the opposite direction, at a velocity v_(diastole) givenby the following expression: $\begin{matrix}{V_{diastole} = \frac{D_{dias} - D_{sys}}{I_{diastole}}} & \text{Eqn.~~5}\end{matrix}$

[0040] However, as shown in FIG. 4 the linear model of FIG. 3 isinaccurate during the diastolic phase generally shown with referencenumeral 130. FIG. 4 shows the data points for eight subjects that depictthe displacement of the right coronary artery (RCA) as a function ofposition in an R-R interval. As shown generally by reference character132, the systolic displacement can generally be estimated as a linearfunction as compared to the more non-linear behavior of the diastolicdisplacement 130. The peak displacements 134 corresponds withend-systole.

[0041] In accordance with the present invention, a cross-correlationalgorithm is used to track the motion of the coronary arteries and theresults are used to prospectively adjust slice position selection andacquisition. A region-of-interest (ROI) that includes the RCA isidentified in the systolic frame and used as a convolution kernel tocompute the cross-correlation data for the entire set. The centralintensity maximum, or peak, in the cross-correlation image essentiallydescribes the motion of the RCA. The position of the central peak andits displacement from the systolic frame position (i.e., the first frameposition) is then automatically computed for each of the desired frames.In a preferred embodiment, the data is then saved to a file and can beused by the desired pulse sequence to prospectively shift the slicelocation to maximize the imaging efficiency. These computations and thefile output take about 1-2 minutes, including image transfer times,resulting in near real-time performance. Moreover, errors due tobreath-holding inconsistencies can be ignored since the displacementsare all relative to the systolic position.

[0042] In the preferred embodiment, a method and apparatus is disclosedfor more accurate, and automated tracking of coronary arteries. Theprocess includes using a multi-phase, single slice scout scan to depictthe vessel of interest in cross-section over a given cardiac cycle. Theprocess then uses image processing techniques to extract the motion ofthe coronary arteries. The information obtained with regard to coronaryartery motion is then used to prospectively adjust the acquisition ofslice positions to improve tracking accuracy. In a preferred embodiment,the right and left coronary arteries are localized using combinations ofsagittal, coronal, and oblique breath-held scout frames obtained usingbreath-held 2D multi-slice fgre sequences. A prerequisite breath-heldCINE scan is first acquired to depict the motion of the cross-section ofthe proximal and distal coronary artery across the cardiac cycle. It isnoted that for the RCA, only the proximal RCA (cross-section) is usedfor the analysis since it provides a much more discemable image than thetypically smaller distal RCA over the entire R-R interval. Allbreath-held images are acquired at end-expiration to improvereproducibility in achieving comparable diaphragm positions from onescan to the next.

[0043]FIG. 5 is a flow chart showing the steps of the present invention.Upon startup 140, the motion of the vessel is then analyzed using the C1images of an imaging plane containing the proximal and distal portionsin cross-section, and then transferring the images to a workstation forpost-processing 142. Preferably, this processing is done using IDLsoftware (Research Systems, Inc. Boulder, Colo.) on an Ultra-Sparc 2Workstation (Sun Microsystems, Mountain View, Calif.) networked to thescanner for transfer of images. The user then delineates aregion-of-interest (ROI) containing the RCA in one frame 144. Typically,this reference frame is chosen as the one in which the RCA is mostdiscernible. A cross-correlation algorithm is then applied between theROI and all the frames to yield a set of correlation maps 146. After thecross-correlation algorithm is applied, the displacement of the centralpeaks relative to the reference frame for each of the frames in thescout scan is then written to a file 150. Accordingly, the positionalinformation of the RCA in the different frames is preferably used tocompute the displacements relative to the systolic position across thecardiac cycle.

[0044] During testing, it was found that the entire process can becompleted in less than a minute, including transfer of the CINE imagesand the correlation analysis. This displacement information is then usedto acquire a series of MR images while prospectively shifting a slicelocation using these non-linear motion parameters between sliceacquisitions. In acquiring the MR data, the displacement information isread from the file and used to acquire and shift the slices depending onthe distance from the reference frame 152. Additionally, the slices maybe reoriented or rotated during the acquisition process to acquire moredesirable images. Preferably, the acquisition of MR data is accomplishedusing a 2D spiral gradient echo pulse sequence with repetition times andflip angles of approximately 100 ms/40°-60° for the vessel trackingsequence and 900-1000 ms/70°-75° flip angle for the multi-slicesequence. Between 12 and 16 interleaves with 2048-4096 points perinterleave were acquired at a receiver bandwidth of ±125 kilohertz. Theaverage breath-holding time for each multi-slice or vessel trackedacquisition was about 15 seconds. All acquisitions were acquired duringend-expiration breath-holds. After reconstructing the images 154, theprocess is complete at 156.

[0045] Referring now to FIG. 6, a graphical representation of sliceacquisition across an R-R interval 160 is shown. In this particularcase, an exemplary eleven slices 162 are acquired over the cardiac cycle160, but each phase is acquired at a different location depending on theposition within the R-R interval 160. The first five slices 164 areacquired during a systolic portion 166 of the R-R interval, and thefinal six slices 168 are acquired during the diastolic portion 170 ofthe R-R interval 160. The systolic and diastolic curves 166, 170 showthe displacement relative to the first frame. FIG. 6 accentuates thedeviation of the actual vessel trajectory 166, 170 from the linear model172, as was described with reference to FIG. 3, particularly, indiastole. The star data points indicate displacements stored in thefile, and the circle data points indicate displacements at slicelocations as computed by interpolation of the data from the file.

[0046] There are many different cross-correlation algorithms that may beused with the present invention. However, one preferredcross-correlation algorithm will be briefly described. In thisalgorithm, once a reference image R is acquired and a firstregion-of-interest ROI is identified in the reference image R, image ROIcan be cross-correlated with each of the other images from a series ofimages to result in a series of cross-correlation images. Thecross-correlation algorithm includes first inverse Fourier transformingthe reference image R within the ROI into a k-space image, flipping thek-space image with respect to its X coordinates, and then flipping thek-space image with respect to its Y coordinates to result in a kernelset K. A mask M is also created having zeros outside of the ROI andone's inside of the ROI. The mask M is then inverse Fourier transformedinto a k-space image and the k-space image of the mask M is flipped withrespect to its X coordinates and then flipped with respect to its Ycoordinates to result in a weighting set W. The kernel set K and theweighting set W are then each multiplied by a raw data image D to resultin a product K, and a product W, respectively. A 2D Fourier transform isthen performed on product K and product W to result in amplitude andphase information (A₂, φ₂), and (A₁, φ₁), respectively. The magnitudes∥₂∥,∥I₁∥ are determined from the amplitude and phase information (A₂,φ₂), and (A₁, φ₁). The cross-correlation image I_(C) is then determinedfrom $I_{C} = {\frac{I_{2}}{I_{1}}.}$

[0047] Since at each location, I₂ is divided by I₁ to result in I_(C)being the intensity of the correlation at that location, atwo-dimensional correlation map is developed. One skilled in the artwill readily recognize such cross-correlation with the initial positionin a region-of-interest with a position in a region-of-interest forsubsequent images can be used to extract the non-linear motionparameters. This is just one example of a cross-correlation algorithm,many others are also applicable.

[0048] Accordingly, the present invention includes a method of imaging amoving object using MRI that includes acquiring a scout image scanhaving a number of image frames, and extracting non-linear motionparameters from the number of image frames from the scout image scan.The method next includes acquiring a series of MR images whileprospectively shifting a slice location using the non-linear motionparameters between slice acquisitions. The motion parameters can includelinear parameters and non-linear parameters, depending upon the objectto be imaged. For example, in coronary artery imaging, the systolicportion of the R-R interval can be estimated using linear parameters,while the diastolic portion can be estimated using non-linearparameters. The method can also include the steps of selecting aregion-of-interest encompassing a coronary artery, recording a referenceposition in the region-of-interest, and then cross-correlating thereference position with a similar position in the region-of-interest forsubsequent image frames to extract the non-linear motion parameters.

[0049] One of the disadvantages of using a vessel tracked sequence isin-plane saturation. For example, if the coronary artery is beingtracked, the same slice is repeatedly excited, necessitating the use ofsmaller flip angles. It is anticipated that the use of a spiral pulsesequence with the technique of the present invention will benefit fromthe use of contrast agents to enable the use of higher flip angles.

[0050] Further, the automatic mapping of the vessel trajectories canprove valuable in reordering k-space for minimizing motion artifacts.Data corresponding to higher k-space values are less sensitive to motioncompared to those from lower, or central k-space. By suitably reorderingk-space data, more data can be acquired in an R-R interval, or motioncan be minimized in the acquired data by using such information. Thiscan then help reduce scan times by improving scan efficiency.

[0051] The present invention also includes a method of coronary MRangiography imaging that includes tracking coronary artery movementduring a cardiac cycle as the coronary artery moves between twoexcursion positions, and determining a function of the coronary arterymovement, where the function includes at least a non-linear componentthat represents movement during a diastolic portion of an R-R interval.The method includes adjusting slice position acquisition using thedetermined function, and acquiring MR data during each such adjustment.MR images can then be reconstructed having increased imaging efficiency.

[0052] Preferably, the MR data is acquired in a single breath-held R-Rinterval. The step of tracking coronary artery movement is accomplishedby acquiring a set of scout frames and developing a non-linear motionfunction based on the relative position of the coronary artery in eachscout frame. A reference kernel derived from an ROI in one scout frameis cross-correlated with the other scout frames to develop thenon-linear motion function. The scout frames are preferably acquiredusing a CINE pulse sequence to track coronary artery movement. A 2Dspiral pulse sequence is used to acquire the MR data based on thedisplacement determined from the scout frames.

[0053] The invention also includes an MRI apparatus to track and image amoving object-of-interest that includes an MRI system, such as thatdescribed with reference to FIG. 1, and a computer, also as describedwith reference to FIG. 1, that is programmed to acquire a series ofscout scans of the object-of-interest using a multi-phase, single sliceacquisition pulse sequence as the object-of-interest moves from oneposition to another. The computer is also programmed to identify areference position in a scout scan, and track movement of theobject-of-interest by isolating the reference position in each scoutscan. A set of motion parameters are determined and stored that are thenused to track movement of the object-of-interest that has non-linearmotion characteristics. The motion parameters are then used to acquireMR data at differing slice locations, each slice location offset from aprevious slice location based on the displacement of the scout scans.

[0054] Accordingly, the present invention also includes a computerprogram comprising instructions which, when executed by a computer,cause the computer to track coronary artery movement during a cardiaccycle as the coronary artery moves between two positions, and determinemovement parameters of coronary artery movement. These movementparameters have at least a non-linear component that represents movementduring a diastolic portion of an R-R interval. The computer program alsocauses the computer to adjust slice position acquisition using thestored movement parameters and acquire image data during each suchadjustment. An image is then reconstructed having increased imagingefficiency using the image data acquired.

[0055] It is noted that this technique can be used for other purposes,such as the tracking of motion in valve planes to help characterizevalvular disease. Such tracking can be conducted in a similar manner asthat described with reference to coronary artery tracking.

[0056] It is noted that all imaging tests were performed on acardiovascular 1.5T MR scanner (CVi, General Electric Medical Systems,Waukesha, WI) equipped with high performance gradients (40 mT/m, 150T/m/s) using a dedicated four-channel phased array coil. Further, whileprevious vessel tracking techniques used a segmented k-space fastgradient recalled echo sequence (fgre), the present invention preferablyemploys a 2D spiral pulse sequence. Spiral sequences have been shown toyield higher SNR images of the coronary artery due to the longersequence repetition times and the higher flip angles. Furthermore, themore efficient coverage of k-space per RF excitation pulse, yields ahigher spatial resolution for the same scan time. Accordingly, in-planespatial resolution of approximately 0.8 mm is achievable using thissequence.

[0057] While not shown, a persistence can be added at end-systole, toaccount for the fact that the heart stays in the end-systolic positionfor 30-70 milliseconds.

[0058] The present invention has been described in terms of thepreferred embodiment, and it is recognized that equivalents,alternatives, and modifications, aside from those expressly stated, arepossible and within the scope of the appending claims.

What is claimed is:
 1. A method of imaging a moving object using MRIcomprising: acquiring a scout image scan having a number of imageframes; extracting non-linear motion parameters from the number of imageframes of the scout image scan; and acquiring a series of MR imageswhile prospectively shifting a slice location using the non-linearmotion parameters between slice acquisitions.
 2. The method of claim 1wherein the motion parameters include linear parameters and non-linearparameters.
 3. The method of claim 1 further comprising the steps of:selecting a region-of-interest encompassing a coronary artery in areference frame; recording a reference position in theregion-of-interest of the reference frame; and cross-correlating thereference position in the region-of-interest of the reference frame withsuch a position in a region-of-interest for subsequent image frames toextract the non-linear motion parameters.
 4. The method of claim 3further comprising the step of computing displacement of the coronaryartery in each frame relative to a reference position.
 5. The method ofclaim 4 wherein the reference position is chosen at an end of diastole.6. The method of claim 1 further comprising the step of using amulti-phase, single slice pulse sequence to acquire the scout imagingscan, and using a gradient echo spiral pulse sequence to acquire theseries of MR images.
 7. The method of claim 1 further comprising thestep of injecting a contrast agent prior to acquiring the series of MRimages to allow use of a pulse sequence with high flip angles.
 8. Themethod of claim 1 wherein the step of acquiring a series of MR imagesincludes data acquisition during a portion of an R-R interval andfurther comprises at least one of the steps of reorienting the slicelocation and rotating the slice location.
 9. A method of coronary MRangiography imaging comprising: tracking coronary artery movement duringa cardiac cycle as the coronary artery moves between two excursionpositions; determining a function of the coronary artery movement havingat least a non-linear component that represents movement during adiastolic portion of an R-R interval; adjusting slice positionacquisition using the determined function; acquiring MR data during eachsuch adjustment; and reconstructing an MR image having increased imagingefficiency using the MR data acquired.
 10. The method of claim 9 whereinthe MR data is acquired in a single breath-held R-R interval.
 11. Themethod of claim 9 further comprising the step of identifying a referencekernel in an ROI in one scout frame.
 12. The method of claim 9 furthercomprising the step of acquiring a set of scout frames which displaymovement of the coronary artery over the cardiac cycle.
 13. The methodof claim 12 further comprising the step of developing a non-linearmotion function based on a position of the coronary artery in each scoutframe.
 14. The method of claim 13 further comprising the step ofcross-correlating a reference kernel from one of the scout frames withthe set of scout frames to develop the non-linear motion function. 15.The method of claim 12 further comprising the steps of: drawing an ROIaround a distal portion of the coronary artery and an ROI around aproximal artery; cross-correlating the distal ROI with each scout framein order to map a relative proximal position of the distal portion ofthe coronary artery in each scout frame; and using the cross-correlationand the relative proximal positions to determine a rotational angle andan offset of each slice in a coronary imaging sequence.
 16. The methodof claim 13 wherein the non-linear motion function includes a linearcomponent associated with a systolic portion of the R-R interval. 17.The method of claim 9 further comprising the step of using a CINE pulsesequence to track coronary artery movement and using a 2D spiral pulsesequence to acquire the MR data.
 18. An MRI apparatus to track and imagea moving object-of-interest comprising: a magnetic resonance imaging(MRI) system having a plurality of gradient coils positioned about thebore of a magnet to impress a polarizing magnet field and an RFtransceiver system and an RF switch controlled by a pulse module totransmit RF signals to an RF coil assembly to acquire MR images; and acomputer programmed to: acquire a series of scout s ans of theobject-of-interest using a multi-phase, single slice acquisition pulsesequence as the object-of-interest moves from one position to another;identify a reference position in a scout scan; track movement of theobject-of-interest by isolating the reference position in each scoutscan of the series of scout scans; and determine and store motionparameters of the movement of the object-of-interest wherein the motionparameters includes non-linear parameters.
 19. The MRI apparatus ofclaim 18 wherein the object-of-interest is a coronary artery and thescout scan acquisition depicts the coronary artery in cross-section overat least a portion of a cardiac cycle.
 20. The MRI apparatus of claim 18wherein the computer is further programmed to use the stored motionparameters to acquire MR images of the object-of-interest as theobject-of-interest moves from the one position to the other by shiftinga slice location for each MR image by an amount determined by the storedmotion parameters.
 21. The MRI apparatus of claim 18 wherein the motionparameters include linear parameters.
 22. The MRI apparatus of claim 19wherein the non-linear parameters depict movement of the coronary arterythrough diastole.
 23. A computer program comprising instructions which,when executed by a computer, cause the computer to: track coronaryartery movement during a cardiac cycle as the coronary artery movesbetween two positions; determine movement parameters of the coronaryartery having at least a non-linear component that represents movementduring a diastolic portion of an R-R interval; store the movementparameters in memory; adjust slice position acquisition using the storedmovement parameters; acquire image data during each such adjustment; andreconstruct an image having increased imaging efficiency using the imagedata acquired.
 24. The computer program of claim 23 wherein the imagedata is acquired by an MRI apparatus and is acquired in a singlebreath-held R-R interval.
 25. The computer program of claim 23 furthercausing the computer to identify a reference kernel in an ROI in onescout frame.
 26. The computer program of claim 25 wherein the act oftracking coronary artery movement includes acquisition of a set of scoutframes with a CINE pulse sequence and wherein the computer programfurther causes the computer to develop a non-linear motion functionbased on a position of the reference position in each scout frame withrespect to the reference kernel.
 27. The computer program of claim 23wherein the computer is further caused to cross-correlate a referencekernel with each scout frame to develop the non-linear motion function.28. The computer program of claim 26 wherein the non-linear motionfunction includes a linear component associated with a systolic portionof the R-R interval and the computer is further caused to use a spiralpulse sequence to acquire MR data.